1. Field of the Invention
The present invention relates generally to a solid state image sensor. The present invention particularly concerns a solid state image sensor having a high S/N ratio and wide dynamic range.
2. Description of the Prior Art
Conventional solid state image sensors can be generally classified into X-Y addressing MOS type image sensors (hereinafter referred to as MOS type) and charge transfer type CCD image sensors (hereinafter referred to as CCD type). It is known that among the above-mentioned two types, the latter-mentioned CCD type is 2-3 times advantageous in sensitivity than the MOS type. This is because, in the CCD type, especially in the interline type which is the representative type, a high S/N ratio is obtainable by decreasing detection capacitances charge detectors built in on-chip in the image sensor. However, as far as this CCD type image sensor is used, even when utmost care is made in decreasing internal noise of the device to improve the S/N ratio, as a result of shot noise theoretically induced by the incident light the S/N ratio of light signal as such has a limit. Therefore, this CCD type is not necessarily absolutely advantageous in achieving high sensitivity. And furthermore, the shot noise due to the input light signal becomes prominent as the degree of integration of devices advances more. Furthermore, in the CCD type device, since BCCD (buried-channel CCD), which is advantageous in transfer efficiency and smallness of noise for vertical transfer parts, is generally utilized, maximum transfer charge can not be made large. And this becomes more problematic when the device is further miniaturized. Furthermore, because of wideness of depletion region and having residual dark current in BCCD even after reading out from photodiodes to the BCCD, undesirable influence of increase of dark current depending on temperature rise is considerable. Accordingly FPN (fixed pattern noise) induced by dispersion of the dark current increases, thereby lowering S/N ratio and the dynamic range.
Another type of solid state image sensor utilizing SIT (static induction transistor) which was reported in the Japan Annual Review in Electronics, Computers and Telecommunications, vol. 8, "Semiconductor Technology" (1983), p. 219-242, has a high sensitivity amplification function. In order to take good advantage of the high sensitivity characteristic of the SIT image sensor, a configuration of solid state image sensor capable of high S/N ratio, but there has been no reports on such study, so far.